The current invention relates to implantable lead or catheter systems; and more particularly, relates to a system and method for detecting when an implanted lead or catheter becomes dislodged from body tissue.
Many modern medical therapies utilize internally-located leads or catheters implanted within the body either acutely or chronically to delivery therapy and/or to perform diagnosis. For example, cardiac pacemakers typically employ endocardial pacing leads that carry electrodes positioned within the atrial and/or ventricular chambers of the heart to deliver electrical stimulation to cardiac tissue. These leads may also be utilized to obtain an electrogram (EGM) waveform which is indicative of the electrical signals occurring within a patient""s heart. Other types of leads carrying various types of sensors may be located within a patient""s heart, vascular system, or at other locations within the body to obtain other physiological signals used in diagnosis and treatment of a patient. For example, leads carrying pressure, temperature, flow-rate, activity, and many other types of sensors may be located within the body to gather physiological data.
Some types of leads or catheters are located within a patient temporarily to perform measurements. For example, commonly-assigned U.S. Pat. No. 5,697,377 to Wittkampf incorporated herein by reference describes a system and method of determining the precise location of a medical device as that device is navigated through the vascular system of a patient""s body. The disclosed navigation system, which may be utilized during mapping, surgical, or implant procedures, employs a reference catheter positioned at a predetermined, stationary position within the patient""s body. The medical device, which may be a lead or catheter, carries a second electrode. The voltage potential difference existing between the reference electrode and the second electrode as described in terms of a three dimensional vector is utilized to determine the precise location of the medical device within the body. To obtain accurate location information, it is critical that the reference catheter be maintained at a stationary position in the body.
In any of the foregoing examples, it is generally considered important to maintain the lead or catheter at a predetermined location within a patient""s body. For example, obtaining meaningful physiological measurements often depends on retaining an instrument at a desired location within a body. Similarly, use of reference leads or electrodes to obtain positional data as performed by the above-described navigational system also requires the ability to maintain the reference lead in a stationary position.
Many types of mechanisms have been developed to aid in retaining implantable devices such as catheters and leads at stationary positions within a body. For example, the body of a lead may be shaped to urge an electrode into contact with predetermined body tissue such as the wall of a vessel or the heart. Such xe2x80x9cpassivexe2x80x9d fixation mechanisms are described in U.S. Pat. No. 4,154,247 issued to O""Neill, U.S. Pat. No. 5,628,778, issued to Kruse et al. and U.S. Pat. No. 5,628,779 issued to Bornzin et al. Other passive fixation mechanisms include the use of tines located on a distal portion of a lead. These tines engage the trabeculae of the heart""s inner surface or the walls of a vessel to stabilize the lead at a predetermined location. Alternatively, xe2x80x9cactivexe2x80x9d fixation mechanisms such as a barb or hook extending from the lead body may be used to engage body tissue and hold the lead in place. The fixation mechanism may also serve as a pacing electrode. For example, a helix may be used to affix a lead to body tissue and to also delivery electrical simulation to the tissue. Such leads are disclosed in U.S. Pat. No. 4,402,329 issued to Williams and to U.S. Pat. No. 4,497,326 issued to Curry.
Although many forms of active and passive fixation mechanisms are known, lead dislodgement remains a problem. For this reason, many systems have been developed to detect catheter or lead dislodgement. One approach to detection of dislocation is set forth in U.S. Pat. No. 5,713,932 issued to Gillberg et al. This patent discloses a cardiac stimulator in which a test pace pulse is delivered to the right atrium. If a ventricular depolarization occurs within a predetermined expected time interval after the pace pulse is delivered, the pace/sense electrode in the atrium is determined to be in contact with cardiac tissue.
According to another method of detecting lead dislodgement as disclosed in U.S. Pat. No. 5,944,746, a change in impedance measurements as measured between multiple electrodes positioned within a heart chamber are utilized to detect lead dislodgement. This mechanism, like the one discussed in the foregoing paragraph, has the disadvantage of requiring multiple electrodes positioned at various locations within the body. Such a configuration may not be available in some situations.
An alternative approach for detecting lead or catheter dislocation is disclosed in U.S. Pat. No. 6,067,469. According to this method, the various characteristics of an electrogram (EGM) are analyzed to determine whether lead dislodgement has occurred. More specifically, the signal levels occurring at the peak of the P and R waves are compared. If the R-wave peak value is a predetermined percentage of the P-wave peak value, lead dislodgement is indicated. This mechanism requires the capability to monitor and analyze an EGM signal. Not all systems include this type of capability.
While many of the existing mechanisms for detecting lead dislodgement are adapted for chronically-implanted lead systems coupled to implantable pulse generation systems, what is needed is an improved method for detecting lead dislodgement adapted for use with acutely-implanted lead or catheter systems not necessarily coupled to a pacing device. Ideally, the system is adapted for use with the type of navigational system described in the above-referenced patent to Wittkampf.
The system and method of the current invention provides an improved mechanism for detecting dislodgement of a distal end of an implantable medical device (IMD) such as a catheter or lead that is implanted within a body. The invention includes an IMD having an affixation device such as a helix at a distal tip. This IMD carries at least two sensing devices such as electrodes located on a portion of the IMD. Preferably, these sensing devices are located near the IMD distal end. The system further includes means for generating multiple signals within the body. In one embodiment, each of the multiple signals is associated with a respective one of the X, Y, or Z directions.
In use, two of the sensing devices carried by the IMD are used to sense the multiple signals that are generated within the body. The difference in signal levels existing between the sensing devices is determined. Because this difference in signal levels has components in the X, Y, and Z directions, this difference may be used to define a directional vector in three-dimensional space. The vector is indicative of the orientation of the IMD. More specifically, this vector substantially corresponds to the orientation of the longitudinal axis of the distal end of the IMD.
One embodiment of the invention includes means to generate three orthogonally-related current signals within body. These current signals result in a voltage potential difference being generated between points within the body. This voltage potential difference, which may be measured between two electrodes, has components in the X, Y, and Z directions. These components may be made distinguishable by providing currents that each have a respectively different frequency, or that are offset from one another by a phase shift, for example. The voltage potential difference signal that may be measured between the two sensing devices may be used to derive a vector indicative of the orientation of the IMD.
In another embodiment of the invention, three magnetic fields may be established having a substantially orthogonal relationship with respect to one another. A device for sensing the strength of the magnetic field such as a Hall effect device may be used to sense the components of a magnetic field in a manner similar to that discussed above. This allows a vector to be obtained that describes the orientation of the IMD.
The inventive system includes means to monitor the orientation of the IMD to detect lead dislodgement. According to one embodiment of the invention, a reference orientation of the IMD is selected. A reference vector associated with this reference orientation is calculated by measuring signal levels between two of the sensing devices in the manner discussed above. Preferably, the reference vector is obtained when the longitudinal axis of the IMD at the point of affixation to tissue is substantially perpendicular to the surface of the tissue.
After a reference vector is derived, subsequent movement of the IMD is monitored by deriving directional vectors associated with new orientations of the IMD. The IMD is considered to be dislodged when a vector associated with a new position of the IMD has a predetermined relationship to the reference vector. For example, in one embodiment, the IMD is considered dislodged when the angle between the reference vector and the newly-derived vector exceeds a predetermined maximum angle. This maximum angle may be selectable by a user so that the sensitivity of the system may be controlled. Selection of the smaller angle provides a system that more readily indicates lead displacement.
The system may include a display to allow a user to view the IMD using, for example, fluorovisible media located at the distal end of the IMD. This may be used to confirm lead dislodgement. The system may further include a user interface to allow a user to select the maximum angle of movement. In one embodiment, the user is further allowed to select the reference orientation of the IMD. An audible alarm may be provided to alert the user to lead dislodgement.
The system described in the fore-going paragraphs is particularly adapted for use with a navigation system of the type discussed in above-referenced U.S. Pat. No. 5,697,377 to Wittkampf. This navigation system tracks the movement of a second IMD as compared to a stationary reference IMD by measuring the difference in signal levels between a point on the second IMD and the reference IMD. This is accomplished using principles similar to those discussed above. The accuracy of this navigation system depends on the reference IMD maintaining a stationary position within a body. It is therefore important that any dislodgement of the reference IMD be detected immediately. This information is readily provided by the current inventive system.